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The horror stories about the Islamic State's mass killings at a cavernous hole in the desert near Mosul became legendary over the years.

Soon after the group took control of the Iraqi city more than 2 1/2 years ago, the 30m-wide sinkhole 8km southwest of the airport became a site for executions. Some victims were made to line up at the edge of the hole and were shot before being kicked inside, while others were tossed in alive, residents said. Sometimes bodies were just trucked in for dumping.

Residents of Mosul whispered about the deaths at the sinkhole, or "khasfa" as it is called. But with communications limited and locals too fearful to speak out publicly, it was only after Iraqi forces retook the area last month as they closed in on the city that the scale of the killing at the site began to emerge. Based on anecdotal evidence, Iraqi officials say thousands may have perished there in recent years.

It may be years more, though, before the mass grave gives up its secrets.

No one knows the depth of the hole under the water at the bottom. The militants have now filled it over and booby-trapped it with explosives, making excavation particularly complex.

Even before the Islamic State's brutal campaign began, Iraqi authorities were struggling to excavate and identify victims of mass graves dating back to the reign of Saddam Hussein, when as many as 1 million Iraqis went missing. Sectarian war following the 2003 US-led invasion brought more large-scale bloodletting.

It is only since Iraqi forces retook the area last month as they closed in on Mosul that the scale of the killing at the sinkhole began to emerge.

Meanwhile, authorities are overwhelmed. Members of Iraq's human rights commission, which is tasked with mapping the Islamic State's mass graves, said they could not provide figures on how many have been found so far. Last summer, the Associated Press said it had documented some 72 mass graves from Islamic State atrocities in Iraq and Syria, containing as many as 15,000 bodies, with more expected to be unearthed.

Dozens of mass graves around the town of Sinjar, which are thought to contain the remains of hundreds of Yazidis executed by the Islamic State, are yet to be fully excavated. Mass graves around the Iraqi city of Tikrit containing the remains of an estimated 1700 soldiers from nearby Camp Speicher who were massacred by the militants, are still being discovered two years after the area was retaken by security forces.

The khasfa, though, could be the group's biggest mass grave.

"It's swallowed the lives of thousands," said Muthanna Ahmed, who said he worked near the site for five months and witnessed executions. "It was terrifying, very deep and dark."

Ahmed said victims' shoes and dried blood lined its rim, while some decaying bodies that got caught on the sinkhole's rugged edge were still visible. A gruesome video posted on YouTube in January 2015 shows a similar scene.

The sinkhole was near an Islamic State oil refinery, and the militants regularly rounded up workers and Mosul residents who were buying fuel to watch the executions. Victims included former police and army officers, as well as those accused of spying or working with the Iraqi government, witnesses said.

Iraqi security forces advance during fighting against Islamic State militants on the western side of Mosul, Iraq. Photo / AP

Hussam al-Abar, a provincial council member, said that between 3000 and 5000 corpses might languish in its depths, though he bases that number on lists of missing people that he concedes could have been killed and buried somewhere else.

"Given the capacity of the central government and local government, I think it's impossible to take out the bodies," he said. "We'd need international assistance. It would be impossible for Iraqis alone."

Before 2003, the sinkhole was a small tourist attraction, drawing travellers from the main Mosul-Baghdad highway 2.5km away, Abar said. But as violence gripped Iraq in the wake of the invasion, al-Qaida began to gain a foothold and the site became a desert grave.

"It was known that whoever wanted to hide a body could drop it in this hole," Abar said.

But it was not until after the Islamic State took control of Mosul in July 2014 that it started being used on an industrial scale.

Jassim Omar, 33, said he witnessed about 10 executions there. The first was about a month after the city fell to the militants. About 25 prisoners from Badush prison in Mosul were brought to the sinkhole and killed, he said.

"If you want to scare someone from Mosul, just mention the khasfa," he said.

The militants killed hundreds of the prison's inmates when they took over the city, according to human rights groups. Most of the victims were Shiites, Yazidis and Christians, all of whom the militants consider to be apostates, while many Sunni inmates were allowed to go free.

In an execution in March or April of 2015, Omar recognised his cousin among a dozen detainees brought to the site in the back of a truck, blindfolded and bound. His cousin had worked for Mosul's local council before the militants took over and was accused of collaborating with the government.

Iraqi forces advance during fighting against Islamic State militants on the western side of Mosul earlier this week. Photo / AP

"Whenever we went, we expected to see executions," he said. "But we were surprised to see our cousin. We just thought he'd been arrested."

He said he watched as the men were made to kneel and three or four militants shot them, while a few others pushed the bodies into the hole.

The stench could be smelled several miles away, he said.

The smell might have been what led the Islamic State to fill the hole in mid-2015. Residents of Athbah, the nearest village, had complained, and some had even left, said Jawad al-Shammari, a spokesman for the human rights committee, which has not yet sent a team to examine the site.

Residents said the Islamic State pushed dozens of trailers and old cars into the hole before filling it with earth, though some said mass killings at the site continued until as recently as six months ago.

Human Rights Watch, which began monitoring the site by satellite in September 2014, said the hole had been filled over by July 2015, though fresh track marks appeared there until November 2016.

"It will take ages for them to decontaminate the site and excavate," said Belkis Wille, senior Iraq researcher for Human Rights Watch. "De-miners are rightly prioritising decontaminating areas that displaced people are returning to."

With explosives planted in the area around it, the khasfa claimed its latest victims last week. Shifa Gardi, a 30-year-old reporter for the Kurdish television channel Rudaw died with a militia commander and four other soldiers when the group set off a booby trap near the site.